As an alternative to printing techniques or systems heretofore used generally, there have been developed ink jet, heat transfer or other systems, which give improved monochromatic or full-color images in a simple and quick manner. Among these, the most excellent is a so-called sublimation type of heat transfer system using a sublimable dye, since it can successfully give a full-color image having an improved continuous gradation and color comparable to a color photograph.
In general, a heat transfer sheet used with the sublimation type of heat transfer system includes a substrate film such as a polyester film which is provided on one side with a dye layer containing a sublimable dye and on the other side with a heat-resistant layer to prevent a thermal head from sticking to the substrate film.
The surface of the dye layer of such a heat transfer sheet is overlaid on an imageable or image-receiving sheet including an image-receiving layer comprising a polyester resin. With a thermal head, the heat transfer sheet is then heated from its back side in an imagewise manner to pass the dye of the dye layer into the imageable sheet, thereby forming a desired image.
The heat transfer system is greatly advantageous in that the density of the image can be controlled by the temperature of the thermal head. However, if the temperature of the thermal head is elevated for a further density increase, then a binder forming the dye layer softens and adheres to the imageable sheet, posing a problem that the heat transfer sheet is bonded to the imageable sheet. If worse comes to worst, the dye layer remains transferred onto the surface of the imageable sheet when it is released from the heat transfer sheet.
An increase in the density of the image may also be achieved by increasing the concentration of the dye in the dye layer. In this case, however, the same problems as mentioned just above arise, since there is a relative decrease in the proportion of the binder contained in the dye layer.
In order to solve such problems, it has been proposed to interpose between the substrate film and the dye layer an adhesive layer comprising an ordinary adhesive resin such as polyurethane or polyester. In general, such an adhesive layer has been formed by coating on the surface of the substrate film a coating solution in which the adhesive resin is dissolved or dispersed in a solvent, followed by drying.
The provision of such an adhesive layer, however, leads to other problems, as set out below:
(1) It is desired that the substrate film and adhesive layer be both reduced in thickness as much as possible in order to keep the sensitivity of the resulting heat transfer sheet in good condition. However, when the substrate film is on the order of, say, a few .mu.m in thickness, it is not easy to coat an adhesive layer coating solution on its surface, making a coating thickness variation likely to occur.
(2) The adhesive layer should also preferably be reduced in thickness as much as possible. To this end, it is required to use a coating solution having a reduced content of solid matter. A problem with the use of such a coating solution, however, is that a large quantity of an organic solvent is consumed in forming the adhesive layer. To make matters worse, a considerable difficulty is encountered in forming a uniform adhesive layer as thin as 1 .mu.m or less.
With the conventional techniques, therefore, it is still unsuccessful to prevent the dye layer from peeling off when the adhesive layer is thin. When the adhesive layer is thick, on the other hand, a sensitivity drop is unavoidable.
It is thus an object of this invention to provide a heat transfer sheet which successfully prevents the dye layer from peeling off at the time of heat transfer and can impart a high density to the image with an improved heat efficiency.